Recording layer for heat assisted magnetic recording

ABSTRACT

A magnetic stack includes multiple granular layers, at least one of the multiple granular layers is a magnetic layer that includes exchange coupled magnetic grains separated by a segregant having Ms greater than 100 emu/cc. Each of the multiple granular layers have anisotropic thermal conductivity.

BACKGROUND

Higher areal storage density for magnetic storage drives can be achievedby decreasing the size of magnetic grains used for magnetic recordingmedia. As grain sizes with a given magnetic anisotropy energy decreasein volume, eventually a stability limit is reached, at which randomthermal fluctuations at room temperature result in magnetizationreversal and corresponding loss of data reliability.

SUMMARY

A magnetic stack includes multiple granular layers, at least one of themultiple granular layers comprises a magnetic layer including exchangecoupled magnetic grains separated by a segregant having Ms greater than100 emu/cc. Each of the multiple granular layers have anisotropicthermal conductivity.

These and other features can be understood in view of the followingdetailed discussion and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional diagram showing a magnetic stack having amultiple layer recording layer with anisotropic thermal conductivity inaccordance with some embodiments;

FIG. 2 is an isometric view of the multiple layer recording layer ofFIG. 1.

FIG. 3 is a cross sectional diagram illustrating a structureincorporating multiple granular layers providing anisotropic thermalconductivity in accordance with some embodiments; and

FIG. 4 is a schematic illustration of a heat assisted magnetic recordingapparatus incorporating a magnetic stack in accordance with someembodiments.

DETAILED DESCRIPTION

To avoid the thermal stability limit, materials having higher magneticanisotropy energy at room temperature can be used to form the magneticgrains of a magnetic stack. However, writing data to these high magneticanisotropy materials is more difficult and some type of energyassistance may be used to temporarily reduce the magnetic anisotropyenergy of the materials while a magnetic write field is applied. Forexample, one form of energy assisted magnetic recording involves laserlight applied to the magnetic medium in synchrony with the writingmagnetic field. The laser light heats the magnetic material to locallylower the magnetic anisotropy of the medium, thus allowing the writingof data by the writing field before the local region cools back toambient temperature. The use of heat assisted magnetic recording (HAMR)involves materials and structures of the magnetic medium that canaccommodate the relatively high temperatures involved in the writingprocess as well as have magnetic properties suitable for HAMR.

High signal to noise ratio (SNR) magnetic recording media, may involvesmall uniformly sized magnetic particles or grains; and a moderatelylow, uniform exchange coupling between the particles or grains. Forlongitudinal, perpendicular and tilted magnetic recording media, anacceptable exchange coupling value may be different, but for each case,a uniform, moderate value may exist between each neighboring grain.

Generally, low exchange coupling is desired so that magnetic switchingof neighboring grains does not become too highly correlated. Reducingexchange coupling decreases the size of the magnetic cluster or magneticswitching unit. Cross-track correlation length and media noise arecorrespondingly reduced. However, near-zero exchange coupling betweenmagnetic grains produces a very low squareness sheared hysteresis loop,a broad switching field distribution, less resistance to selfdemagnetization and thermal decay, and low nucleation field (Hn) inperpendicular media designs. Non-uniform exchange coupling allows somegrains to act independently while other grains act in clusters,resulting in broad distributions of magnetic cluster size and anisotropyfield.

Adjusting deposition parameters in an attempt to achieve a recordinglayer that has an amount of non-ferromagnetic material that issufficient to significantly, but not completely exchange decouple themagnetic particles can be challenging. One problem arises because theexchange between two grains is extremely sensitive to the arrangement ofa very small number of atoms. Thus, some grains are much more stronglycoupled than others. Additionally, radial diffusion profiles depend uponthe size of the grains. Thus, larger grains can have systematicallydifferent composition than smaller grains, and hence have systematicallydifferent exchange coupling and magnetic anisotropy. Furthermore, thecomposition of the entire film including the ferromagnetic particles,the weakly exchange coupled ferromagnetic regions between magneticparticles, and the exchange decoupling non-ferromagnetic regions betweenmagnetic particles is substantially the same, except for thepreferential transport of some atomic species. Changing exchangecoupling in the film generally changes the composition of the magneticparticles as well as the grain boundary material. Thus, it is difficultto separately optimize the properties of each component of the film.

Embodiments of the invention comprise a magnetic recording mediasuitable for HAMR applications that includes a magnetic recording layerthat has multiple magnetic layers. One of the multiple layers is morestrongly exchange coupled than another of the multiple layers. Bothmagnetic layers comprise crystalline grains separated by segregantmaterial. The magnetic layer that has weaker exchange coupling hassufficient non-ferromagnetic segregant between magnetic grains so thatexchange coupling is relatively low. The magnetic layer that is morestrongly exchanged coupled has relatively high exchange coupling suchthat substantially all of the magnetic grains are strongly exchangecoupled. The strongly exchange coupled magnetic layer has aferromagnetic segregant between the magnetic grains.

In some implementations, the strongly exchange coupled and weaklyexchanged coupled magnetic layers may be adjacent in the film stack sothat grains in the weakly exchanged coupled magnetic layer aremoderately exchange coupled by a pathway through the strongly exchangedcoupled magnetic layer. That is, exchange coupling between grains in theweakly coupled magnetic layer is increased toward a moderate value bytheir coupling connection through the strongly coupled magnetic layer.

When used in conjunction with HAMR, these magnetic recording layers haveadditional constraints of accommodating local heating and reducing heattransfer to neighboring regions which may disturb previously writtendata. Embodiments discussed in this disclosure involve structures andmaterials configured to provide switching field, switching fielddistribution, thermal stability, heat transfer and other characteristicswithin acceptable limits for HAMR media. FIG. 1 illustrates a crosssectional view of a magnetic stack in accordance with some embodiments.The magnetic stack includes a substrate 110 and may optionally includeone or more interlayers 120 between the substrate 110 and the magneticrecording layer 130. One or more overcoat layers 140 may optionally bedisposed on the magnetic recording layer 130.

The substrate 110 may be made of any suitable material, such as ceramicglass, amorphous glass, or NiP coated Al—Mg alloy. The one or moreinterlayers 120 may include an orientation layer, a heat sink layer,and/or a soft underlayer (SUL). If a heat sink is used, it may have athickness from about 10 to about 1.00 nm, and may be made of anysuitable material, such as Cu, Ag, Al, Au, CuZr, or other materialhaving high thermal conductivity. If an SUL is used, it may have athickness from about 5 to about 50 nm, and may be made of any suitablesoft magnetic material. Some representative, non-limiting SUL materialsare alloys including CoFe, FeCoB, FeAlN, FeAlSi, NiFe, CoZrNb or FeTaN.The heat sink layer, SUL, orientation layer and/or overcoat may comprisemultiple layers.

The magnetic recording layer comprises at least two layers 131, 132. Thefirst magnetic recording layer 131 is a granular structure comprisingfirst magnetic grains or particles 133 and a first segregant 135 at thegrain boundaries of the first magnetic grains 133. The term “granularlayer” denotes an arrangement of crystalline cores with grainboundaries, wherein the crystalline cores are separated from one anotherby a material at the grain boundaries. A granular layer comprises a twophase structure with the crystalline cores as a first phase and thematerial that separates the crystalline cores as a second phase. Thefirst phase has a physically distinct microstructure or composition whencompared to the second phase material, which may be amorphous,polycrystalline, and/or crystalline. The second magnetic recording layer132 is also a granular structure comprising second magnetic grains orparticles 134 and a second segregant 136 at the grain boundaries of thesecond magnetic grains 134. The first segregant 135 has low or nomagnetic moment, and magnetically decouples the first magnetic grains133 of the first magnetic layer 131 from each other. The secondsegregant 136 has higher magnetic moment and may be thinner than firstsegregant 135, and does not fully magnetically decouple the secondmagnetic grains 134 of the second magnetic layer 132 from each other.Any decoupling provided by the second segregant 136 for the secondmagnetic grains 134 is less than the decoupling provided by the firstsegregant 135 for the first magnetic grains 133. For example, the secondsegregant 136 may provide negligible decoupling between the secondmagnetic grains 134. The first and/or second segregant may be anamorphous material. The term “amorphous” means that such a materialexhibits no sharp peak in a θ-2 θ X-ray diffraction pattern as comparedto background noise. Amorphous materials may encompass nanocrystallitesin amorphous phase or any other form of a material so long the materialexhibits no peak in an X-ray diffraction pattern as compared tobackground noise.

The amount of lateral exchange decoupling between grains within thefirst and second recording layers 131, 132 may differ. For example, asdiscussed in an example below, the grains 134 of the second magneticrecording layer 132, which in FIG. 1 is farthest from the substrate 110,may be less exchange decoupled than the grains 133 of the first magneticrecording layer 131. However, in other implementations, the reversesituation may hold, wherein the grains of the second recording layer aremore exchange decoupled than the grains of the first recording layer.

The amount of exchange coupling between the magnetic grains of amagnetic layer may be described in terms of an exchange field (Hex)which is a measure of the magnetic interaction between neighboringgrains. In some embodiments, the first magnetic layer 131 has anexchange field, Hex₁, that is different from the exchange field of thesecond magnetic layer, Hex₂. For example, the first magnetic layer maybe more exchange decoupled than the second magnetic layer, such thatHex₁<Hex₂.

Completely decoupled grains would have an exchange field, Hex=0 Oe, andhave a highly sheared hysteresis loop owing to the demagnetizationfield, (1−N)*4*π*Ms, where N is the self demagnetization factor and Msis the saturation moment. Thus, for Hex=0, the sheared hysteresis loopmakes the nucleation field (Hn) much smaller than the coercive field(Hc) so that the quantity Hc−Hn is large. Media with strongly coupledgrains all tend to reverse together at about the same applied field, sothat the hysteresis loop is very square and Hc−Hn is small. Largervalues of the quantity Hc−Hn correspond to lower exchange coupling. Anapproximation of exchange field is given by the equationHex=((1−N)*4*π*Ms)−(Hc−Hn). N has a value between 0.1 and 0.5 for mostHAMR media designs. Values of Hc, Hn and Hex provided in examples beloware room temperature values.

The approximate shape of a hysteresis loop including Hc−Hn of adecoupled granular layer with Hex˜0 can be estimated from basic medialayer parameters including thickness, grain size, and Ms. Decoupledlayer Hc−Hn values may vary from less than 5 kOe to more than 15 kOe. Ineach case, for the specified basic media average layer parameters, Hc−Hnis reduced from the decoupled granular layer value as the percentage orthickness of coupled second layer material is increased, thereby raisingthe average Hex of the composite structure.

In one example, the first magnetic layer 131 has first magnetic grains133 separated by a first segregant 135 with comprises a non-magneticoxide. The first magnetic layer 131 has Hc of about 30 kOe and Hn ofabout 20 kOe, resulting in an Hc−Hn of about 10 kOe. In this example,the first magnetic layer 131 has a thickness of about 8 nm, with grainsizes suitable for HAMR applications, e.g., grain diameters of about 2nm to about 15 nm. The second magnetic layer 132 has second magneticgrains 134 separated by a magnetic oxide 136. The second magnetic layer132 may have a thickness of about 3 nm with grain diameters of about 2nm to about 15 nm, and stronger exchange coupling between the grains 134when compared to the exchange coupling between the grains 133 of thefirst magnetic layer 131. This example arrangement of first and secondmagnetic layers 131, 133 may reduce Hc−Hn of the magnetic recordinglayer 130 to about 5 kOe, for example.

In other embodiments, a wide range of coercivities and loop shearinglevels are consistent with intermediate Hex values formed by acombination of one magnetic layer that is an exchange decoupled layerhaving a relatively low Hex value and another magnetic layer having arelatively high Hex value.

In one example, the first magnetic layer 131 includes first magneticgrains 133 separated by a first segregant 135 with comprises anon-magnetic oxide. The Hex value of the first magnetic layer 131, Hex₁,is below about 2 kOe. The second magnetic layer 132 includes secondmagnetic grains 134 separated by a second segregant 136 which comprisesa magnetic oxide. The second magnetic layer 132 has a Hex value, Hex₂,which is larger than Hex₁. The combination of the first and secondmagnetic layers 131, 132 in this example provides a combined magneticlayer having an intermediate Hex value between 3 kOe-10 kOe.

In various implementations, the more decoupled layer having thenon-magnetic segregant has Hex less than about 4 kOe, or even less thanabout 3 kOe, and the more coupled layer having the magnetic segreganthas Hex greater than about 4 kOe, or greater than 5 kOe, or even greaterthan 10 kOe.

Exchange field, Hex, of a magnetic layer is a function of the magneticproperties of the segregant. For example, with other parameters beingheld equal, a segregant material that has a relatively higher magneticmoment, Ms, provides more exchange coupling than a material that has arelatively lower magnetic moment. In some cases, the magnetic moment ofthe non-magnetic segregant 135 may be negligible (e.g., less than about100 emu/cc) whereas the magnetic moment of the magnetic segregant 136may be greater than about 100 emu/cc.

Examples discussed above involve the arrangement in which the firstmagnetic layer has a non-magnetic segregant and is more decoupled thanthe second magnetic layer, which has a magnetic segregant. In somecases, the opposite arrangement may occur. The grains 133 of the firstmagnetic layer 131 may be separated by a magnetic segregant 135 and thegrains 134 of the second magnetic layer 132 may be separated by anon-magnetic segregant 136 such that the first magnetic layer 131 is themore magnetically coupled layer and the second magnetic layer 132 isless magnetically coupled.

In either case, there is vertical interlayer magnetic coupling betweenthe grains of the first layer and those of the second layer. Thisinterlayer coupling serves to increase the effective volume of thegrains, thereby increasing thermal stability of layers 131, 132. Theinterlayer coupling allows the grains to switch more coherently, therebyreducing the SFD of the magnetic stack.

The grains of the first magnetic layer 131 and/or the second magneticlayer 132 may comprise FePt, FeXPt alloy, FeXPd alloy, Co₃Pt, CoXPtalloy, CoX/Pt multilayer, CoXPd alloy, CoX/Pt multilayer, 3D transitionmetal-rare earth alloys such as CoSm alloys, and/or other materials. Thegrains of materials such as FePt, FeXPt alloy, FeXPd alloy, Co₃Pt, CoXPtalloy, and CoXPd alloy may comprise ordered alloys to increase theirroom temperature anisotropy. The segregant 135 of the first magneticlayer 131 comprises a non-magnetic material, such as carbon or an alloycontaining carbon, an alloy containing boron, a non-magnetic oxide,e.g., oxides that do not include a substantially amount of Co, Ni or Fe.For example, C, SiO₂, TiO₂, Ta₂O₅, BN, B₄C, MgO, silicon carbide,silicon nitride, etc. may be used as a non-magnetic segregant. Thesegregant 136 of the second magnetic layer 132 comprises a magneticmaterial, such as magnetic oxides and/or nitrides, e.g., magnetic oxidesor nitrides of iron, nickel or cobalt. For example, FexOy or FexNy maybe used as a magnetic segregant, where x and y represent the relativeamounts of each component in the material.

The first magnetic layer 131 and/or the second magnetic layer may have athickness in a range between about 1 nm and about 15 nm, or a range ofabout 4 nm to about 10 nm, or a range of about 3 to about 8 nm. Theaverage diameter of the grains of the first and/or second magneticlayers may range from about 2 nm to about 15 nm or may range from about5 nm to about 10 nm. The amount of magnetic segregant in the secondmagnetic layer may be in a range of about 5% to about 50% by volume, orin a range of about 15% to about 35%. The amount of non-magneticsegregant in the first magnetic layer may be in a range of about 5% toabout 50% by volume, or in a range of about 15% to about 35%. Theaverage thickness of the segregant between the magnetic grains in thefirst and/or second magnetic layer may be in a range of about 0.5 nm toabout 2.0 nm.

The materials used in the first and second magnetic layers 131, 132 maybe selected so that the thermal conductivies of the layers 131, 132 areanisotropic. During a HAMR write operation, lateral heat flow away fromthe heated area being written can cause errors in previously writtendata in areas of the stack adjacent to the heated area due to thermallyinduced magnetization reversals in adjacent areas. The thermalproperties of the segregants 135, 136 may be selected to control thelateral heat flow, also known as thermal blooming, propagating from theheated area in HAMR.

FIG. 2 is an isometric view of the dual magnetic layers 131, 132presented here to facilitate discussion of the thermal properties ofthese layers. Note that FIG. 2, like others in this disclosure is not toscale and provides an exaggerated view of the recording layers toillustrate certain aspects. Each layer 131, 132 includes grains 133,134, which are represented in FIG. 2 by the shaded regions that extendthrough the thickness of the layers 131, 132. As shown in FIG. 2, thegrains 133 of the first layer 131 may generally align with the grains134 of the second layer 132, although this is not always the case.Segregant material 135, 136 represented by non-shaded regions in FIG. 2,is disposed in between the first layer grains and the second layergrains. The thermal conductivity of the segregant material 135, 136 islower than the thermal conductivity of the grains, resulting inanisotropy in the thermal conductivity of the recording layers 131, 132.Heat transfer from the heated area 150 near the laser spot in the x andy directions is impeded by the relatively low thermal conductivity ofthe segregant material 135, 136, whereas heat transfer in the zdirection occurs through the relatively high thermal conductivity grains133, 134.

Thermal conductivity of the segregant 135 of the first layer 131 and thesegregant 136 of the second layer 132 may be less than about 5 Watts/mK.Thermal conductivity of the magnetic grains 133, 134 may be greater than5 Watts/mK, or greater than about 10 Watts/mK. Along the x and ydirections, in addition to the material thermal conductivities of thesegregant and magnetic grains that make up the magnetic layers, thethermal conductivities of the first and second magnetic layers can alsobe affected by the contact resistance between the segregant and themetal alloy of the magnetic grains. The overall thermal conductivity ofthe first and second magnetic layers in the x or y directions may beless than about 5 Watts/mK. The overall thermal conductivity of thefirst and second magnetic layers in the z direction is predominantlydetermined by the thermal conductivities of the magnetic grains 133,134, which may be greater than about 5 Watts/mK or greater than about 10Watts/mK.

In some implementations, the magnetic stack may include one or moreadditional magnetic or non-magnetic layers, as illustrated by layer 300in FIG. 3. In FIG. 3, layer 300 is shown disposed between the first andsecond magnetic layers. Alternatively or additionally, the one or moreadditional layers may be disposed over the first and second magneticlayers 131, 132 (farther from the substrate 110), and/or may be disposedunder the first and second magnetic layers 131, 132 (nearer to thesubstrate 110). The one or more additional layers 300 comprise amaterial composition and/or structure providing anisotropic thermalconductivity with preferential heat flow normal to the surface of themagnetic stack (z direction) and limited lateral heat flow (x and ydirections).

Turning back to FIG. 3, regardless of their specific location in thestack, the one or more additional layers 300 may have a granularstructure with relatively high thermal conductivity grains 310 separatedby relatively lower thermal conductivity segregant 311. The grains 310of the additional layers 300 may be magnetically coupled or decoupledand the grains 310 and/or segregant 311 may comprise magnetic ornon-magnetic materials.

As shown in FIG. 3, the additional layer 300 is compatiblecrystallographically with the magnetic layers 131, 132 above and belowthe additional layer 300. In this arrangement, additional layer 300 mayassist in initiating good crystallographic growth of the second magneticlayer 132. In various implementations, the additional magnetic layer 300may be magnetic or non-magnetic and does not compromise the verticalcoupling between the first and second magnetic layers 131, 132. Whenlocated between the first and second magnetic layers 131, 132 as shownin FIG. 3, the additional magnetic layer 300 may be sufficiently thinand/or may have sufficient Ms so that the vertical coupling between thefirst and second magnetic layers 131, 132 is not eliminated.

FIG. 4 illustrates a recording apparatus that incorporates one or moreof the magnetic stacks illustrated herein. FIG. 4 provides a schematicside view of a HAMR recording head 422 and a magnetic recording medium416. Although an embodiment of the invention is described herein withreference to recording head 422 as a perpendicular magnetic recordinghead and the medium 416 as a perpendicular magnetic recording medium, itwill be appreciated that aspects of the invention may also be used inconjunction with other types of recording heads and/or recording mediumswhere it may be desirable to employ heat assisted recording. The head422 may include a writer section comprising a main write pole 430 and areturn or opposing pole 432 that are magnetically coupled by a yoke orpedestal 435. The head 422 may be constructed with a write pole 430 onlyand no return pole 432 or yoke 435. A magnetization coil 433 maysurround the yoke or pedestal 435 for energizing the head 422. The HAMRhead 422 may also include a read portion, not shown. The recordingmedium 416 is positioned adjacent to or under the recording head 422.Relative movement occurs between the head and the medium 416.

As illustrated in FIG. 4, the recording head 422 also includes astructure 421 for heating the magnetic recording medium 416 proximate towhere the write pole 430 applies the magnetic write field H to therecording medium 416. The medium 416 includes a substrate 438, one ormore interlayers, e.g., heat sink layer 440, seed layer 441, a magneticrecording layer 442, and an overcoat/protective layer 443. A magneticfield H produced by current in the coil 433 is used to control thedirection of magnetization of bits 444 in the recording layer of themedium.

The structure 421 for heating the medium 416 may include, for example, aplanar optical waveguide 450. A laser diode 452, or other source ofelectromagnetic radiation, directs light 454 to the planar waveguide 450which transmits the light to a small spot adjacent to an air bearingsurface (ABS). A near-field transducer (NFT) can be included to furtherconcentrate the light. The near-field transducer is designed to reach alocal surface plasmon (LSP) condition at a designated light wavelength.At LSP, a high field surrounding the near-field transducer appears, dueto collective oscillation of electrons in the metal. Part of the fieldwill tunnel into an area 460 of the adjacent media and get absorbed,raising the temperature of the media locally for recording.

The examples provided in this disclosure illustrate magnetic stacks thatinclude recording layers having multiple magnetic layers in which eachlayer individually (and all collectively) have anisotropic thermalconductivity. Some of the layers may be granular in structure withexchange coupling between the grains. Some of the layers may be granularin structure with substantially exchange decoupled grains. The conceptcan be extended such that a majority or substantially all of themagnetic layers (recording layers or interlayers) in the stack above aheat sink layer may exhibit anisotropic thermal conductivity. Theselayers can have relatively high thermal conductivity in a directionnormal to the surface of the magnetic stack but have relatively limitedthermal conductivity in the plane of the magnetic stack. Such structuresserve to reduce heat transfer from the heated area of the stack toadjacent areas of the recording layer, preventing transferred heat fromdisturbing previously written data in adjacent areas.

It is to be understood that even though numerous characteristics ofvarious embodiments have been set forth in the foregoing description,together with details of the structure and function of variousembodiments, this detailed description is illustrative only, and changesmay be made in detail, especially in matters of structure andarrangements of parts illustrated by the various embodiments to the fullextent indicated by the broad general meaning of the terms in which theappended claims are expressed.

What is claimed is:
 1. A magnetic stack suitable for use in magneticstorage, comprising: a first granular magnetic layer including firstmagnetic grains separated by a first segregant, the first segregantcomprising a non-magnetic material; and a second granular magnetic layerincluding second magnetic grains separated by a second segregant, thesecond segregant comprising a magnetic material, wherein the secondsegregant has thermal conductivity less than about 5 Watts/mK, and thefirst and second magnetic layers have anisotropic thermal conductivity.2. The magnetic stack of claim 1, wherein the second segregant has M_(s)greater than about 100 emu/cc.
 3. The magnetic stack of claim 1, whereinat least one of the first magnetic grains and the second magnetic grainscomprise an Fe alloy.
 4. The magnetic stack of claim 1, wherein thesecond segregant comprises an oxide or nitride of Fe, Ni or Co.
 5. Themagnetic stack of claim 1, wherein the first segregant comprises carbonor a non-magnetic oxide.
 6. The magnetic stack of claim 1, wherein anexchange field, Hex₁, of the first magnetic layer is less than anexchange field, Hex₂, of the second magnetic layer.
 7. The magneticstack of claim 1, wherein the first and second magnetic grains havediameters less than about 15 nm.
 8. The magnetic stack of claim 1,wherein a component of the thermal conductivity of the first and secondmagnetic layers normal to a surface of the magnetic stack is greaterthan about 5 Watts/mK and a component of the thermal conductivity of thefirst and second magnetic layers parallel to the surface of the magneticstack is less than about 5 Watts/mK.
 9. A magnetic medium suitable foruse in magnetic storage, comprising: a first magnetic layer comprisingfirst magnetic grains separated by a first segregant having Ms less thanabout 100 emu/cc; and a second magnetic layer comprising second magneticgrains separated by a second segregant having thermal conductivity lessthan about 5 Watts/mK and Ms greater than about 100 emu/cc, wherein athermal conductivity component of the first and second magnetic layersnormal to a surface of the magnetic medium is greater than a thermalconductivity component of the first and second magnetic layers parallelto the surface of the medium.
 10. The magnetic medium of claim 9,wherein the thermal conductivity component normal to the surface of themedium is greater than about 10 Watts/mK and the thermal conductivitycomponent parallel to the surface of the medium is less than about 5watts/mK.
 11. The magnetic medium of claim 9, wherein the thermalconductivity component normal to the surface of the medium is greaterthan about 5 Watts/mK and the thermal conductivity component parallel tothe surface of the medium is less than about 5 watts/mK.
 12. Themagnetic medium of claim 9, wherein a thickness of one or both of thefirst and second magnetic layers is in a range of about 3 nm to about 8nm.
 13. A magnetic stack suitable for use in magnetic storage,comprising multiple granular layers, at least one of the multiplegranular layers comprising a magnetic layer including exchange coupledmagnetic grains separated by a segregant having thermal conductivityless than about 5 Watts/mK and Ms greater than 100 emu/cc, each of themultiple granular layers having anisotropic thermal conductivity. 14.The magnetic stack of claim 13, wherein at least one of the multiplegranular layers comprises a magnetic layer including exchange decoupledmagnetic grains separated by a segregant having Ms less than 100 emu/cc.15. The magnetic stack of claim 14, wherein at least one of the multiplegranular layers comprises an additional granular layer disposed betweenthe magnetic layer having the exchange decoupled magnetic grains and themagnetic layer having the exchange coupled magnetic grains.
 16. Themagnetic stack of claim 13, wherein a thermal conductivity componentnormal to a surface of the stack is greater than about 10 Watts/mK and athermal conductivity component parallel to a surface of the stack isless than about 5 watts/mK.
 17. The magnetic stack of claim 13, whereina thermal conductivity component normal to a surface of the stack isgreater than about 5 Watts/mK and a thermal conductivity componentparallel to a surface of the stack is less than about 5 watts/mK. 18.The magnetic stack of claim 13, wherein the segregant comprises an oxideof Fe, Ni or Co.
 19. The magnetic stack of claim 13, wherein thesegregant comprises a nitride of Fe, Ni or Co.
 20. The magnetic stack ofclaim 13, wherein the magnetic grains have diameters less than about 15nm.